Safety system for visual approach

ABSTRACT

An improved safety system for visual approach slope indicator installations at airports is described which does not disable the installation when transient displacements of the key visual elements occur but does disable the installation when long term displacements of the key visual elements occur in such magnitude as to make dangerous reliance on the installation by the pilot of an approaching aircraft.

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Knudsen 1 ay 22, 1973 SAFETY SYSTEM FOR VISUAL [56] References CitedAPPROACH UNITED STATES PATENTS [75] Inventor: Clarence B. Knudson,Inglewood,

C lif 3,701,969 10/1972 Lambert et a1 ..340 25 Assignee: g and Phillips,Burbank, Primary Examiner-Kathleen H. Claffy Cahf- AssistantExaminer-Thomas L. Kundert 22 Filed: Nov. 1971 Attorney-Bruce L.Birchard App]. No; 198,509

US. Cl ..340/ Int. Cl. .13641 1/18, G08g 5/02 Field of Search ..340/25,26; ZOO/61.51, 61.52; 317/141 R; 307/141,

[ 5 7 ABSTRACT An improved safety system for visual approach slopeindicator installations at airports is described which does not disablethe installation when transient displacements of the key visual elementsoccur but does disable the installation when long term displacements ofthe key visual elements occur in such magnitude as to make dangerousreliance on the installation by the pilot of an approaching aircraft.

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INVENTOR. (ILA/25d; B. K/uuoso g BY SAFETY SYSTEM FOR VISUAL APPROACHRELEVANT COPENDING APPLICATION Application Ser. No. 198,506 filed Nov.15, 1971 entitled lntensity Control Circuit for Visual Approach SlopeIndicator Installation.

BACKGROUND OF THE INVENTION To maintain and improve the high standardsof safety in the aeronautics industry of the United States the FederalAviation Agency, which is charged with the safety of air travel in theUnited States, has approved and required the installation of an aircraftapproach system designated by the acronym VASI. VASI stands for VisualApproach Slope Indicator. It is a visual system supplementing ILS(Instrument Landing System) for those aircraft equipped with ILSreceivers and indicators and supplying an accurate slope indicator forthose aircraft, usually the privately owned aircraft, having noelectronic landing approach slope indicating equipment. For the smallerairports with limited traffic an alternative but related system calledSAVASI is available. The first two letters of that acronym stand forSimple Abbreviated.

The principle of operation of VASI and SAVASI, although not directlypart of this invention requires some explanation to appreciate the fullimport of the invention claimed herein. The full details of VASI can befound in Handbook 6850.2 published by the Federal AviationAdministration, U. S. Department of Transportation. In its simplest formtwo light sources (called bars) are located adjacent to but at a safedistance from the center line of the runway to be used and are spacedalong the runway at critical distances to be described. The bar nearestthe landing threshold of the runway is called the downwind bar, and thatfurther along the runway is described as the upwind bar.

Each bar includes one or more lamp housings. Each lamp housing in a barcontains three 200 watt 6.6 ampere pre-focussed halogen cycle lamps andthree sets of red and white spread lenses which produce well defined redand white beams of light having known half-power widths in a verticalplane and in the horizontal plane. For example, the half-power width ofthe vertical radiation pattern for the white beam is approximately 2.The crossover point between the white and red beams is alsopre-determined and set with great accuracy.

The white beam lies abovethe red beam in both the upwind and downwindlight bars. This is achieved by the spread lens filter in front of eachlamp in the light bar. The upper portion of each such lens is red andthe lower portion is clear. The lenses also spread the lighthorizontally.

In addition to the critical location of the upwind and downwind barsalong the runway the aiming of these bars is critical. As has been notedthe light beams are narrow and in order to provide a safe glide slopepath that can be relied upon by a pilot making a landing approach thecorridor must be accurately defined.

When the positioning and aiming of the light bars are correct and thepilot is on the right glide path the downwind bar or bars will appearwhite while the upwind bars appear red. If the approach is too high,both downwind and upwind bars appear white, while an approach which istoo low results in both bars appearing red.

To prevent the generation of erroneous descent information and possibledeath or injury to those aboard an aircraft the Federal AviationAdministration has set some very stringent specifications on the aimingof the light bars and has further defined acceptable tolerances. Theyfurther have required that the light bars be equipped with tilt-switchesthat will disable both light bars in a two bar (one lamp box per bar)system in the event the aiming of either bar changes by /2 below or 1above the correct pre-set aiming angle.

Deviations from the pre-set aiming angle of a light bar can be of a longterm or short term nature. For example, if the light bar is physicallystruck, as by a field maintenance vehicle, the bar may suffer permanentdisplacement and the aiming angle may be permanently lost untilre-setting is accomplished. This is the type of error the tilt-switch isintended to eliminate. On the other hand, temporary deviation of thelight bar from the proper aiming angle may result from the shock wavesproduced by jet engine operation in close proximity to the light bar.This transient type of deviation should not produce shut-down of theVASI system. The FAA has set the vibration which must be tolerated bythe VASI safety system without breaking the light bar electrical circuitas follows. The safety system must withstand vibration of 1 inchamplitude at frequencies from one-half cycle per second to 10 cycles persecond for 10 minutes without shutting ofl' power to the light bars. Atthe same time a sustained tilt for a maximum of 1.5 minutes must shutoff the power to the VASI light bars. The FAA specifications furtherprovide that the safety system shall operate from 55 to +55 C. Further,it is desirable that the tilt-sensing system operate from normallyclosed contacts so as to be fail-safe.

Various methods have been proposed to provide the safety system requiredby the FAA for VASI. For example, it has been proposed that a pendulumbe used which will move from a neutral position corresponding to properlight bar tilt angle to a deviated position when the light bar tiltangle deviates from its pre-set position and such pendulum causes theclosing of electrical contacts which deactivate the VASI system. Such anapproach would not meet the vibration tolerance specifications set bythe F AA nor the desired fail-safe requirements.

In the safety system which constitutes the present invention a pair ofthallium-doped mercury switches is incorporated in adjustable fashion ineach of the light bar housings. The purpose of the thallium in themercury is to lower the freezing point of the mercury so that the systemmeets the 55 C specification set by the FAA. This is a well establishedtechnique and, by itself, does not constitute the invention claimedherein.

The two mercury switches are individually adjusted so that when thehousing is level and the light beams are properly aimed the switchcontacts are closed but when the light bar is tilted more than 1 upwardplus 0 minus W or tilts downward V2 plus 0 minus 56 one or the other ofthe switches opens which would disable the VASI system instantaneouslyexcept for the novel associated electronic circuit which delays suchdisabling until it is clear that the light bar tilting is long term incharacter. The safety system will operate if either the upwind ordownwind light bar is permanently tilted and the VASI system will bedisabled. However, tilting or vibration of either of those light bars bywind gusts, jet blasts or any other transient phenomena will not disablethe VASI system.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT In FIG. 1,tilt-switch module is shown mounted in selectively rotatable position tomounting plate 11 which may be one panel of a lamp housing, by means ofpivot 12 and wing-nut 13 which is applied to threaded bolt 14. Threadedbolt 14 has one end secured to tilt-switch module 10 and passes througharched slot 20 (FIG. 2) in mounting plate 11. Clipmounting bar 15 hasfulcrum screw 16 passing centrally therethrough and securing bar 15 insnugly rotatable fashion to backplate 17. Clip 18 is secured in fixedfashion to mounting bar 15 by means of screw 19.

Similary, in FIG. 2 clips 18, 21, 22 and 23 are fixed to mounting bars15 and 24, respectively. Mercurythallium switches 25 and 26 are securedin removable fashion but with fixed orientation in clips 18, 21 and 22,23, respectively.

L-bar 27 which is secured with fixed orientation to backplate 17 bymeans of screw 28, 29, 30 and 31 carries pre-tilt adjustment screws 32,33, 34 and 35. Mercury-thallium switch 25 includes electrodes 36 and 37,and mercury-thallium mass 38. Mercurythallium switch 26 includeselectrodes 39 and 40, and mercurythallium mass 41. Output leads 42 and43 from mercury-thallium switches 25 and 26, respectively, are connectedtogether and to conductor 44. Output leads 45 and 46 from switches 25and 26, respectively, are connected to conductors 47 and 48,respectively. Conductor 44 is connected to junction 79 of resistors 49and 50. Conductor 47 is connected to junction 78 of resistor 51 andcondensor 52 which shunt pull-in winding 53 of relay 54. Conductor 48 isconnected to junction 77 of resistor 55 and condenser 56 which shutpullin winding 57 of relay 58. Normally open contacts 59 and 60 ofrelays 54 and 58, respectively, are connected in series with pull-incoil 61 of power relay 62 across the power source, which in this case isa 120 volt A. C. line. VASI lamps 63 are connected in series withnormally open relay contacts 64 across terminals 65 and 66 which areconnected to a lamp power source such as that described in copendingapplication Ser. No. 198,506 in the name of Robert H. Homer, assigned tothe same assignee as this application. D. C. power for operating relays58 and 54 is provided from power supply 67 comprising resistor 68,diodes 69, 70, 71 and 72 connected in a bridge configuration and filtercapacitor 73 in combination with resistors 49 and 50. A. C. power to berectified is applied across the bridge rectifier through conductors 74and 75 and resistor 68.

The safety system of the present invention operates as follows. At theoutset the VASI lamps 63, which may be in the same external housing asthe tilt-switch module 10, are oriented horizontally and pre-tilted oraimed in the elevation plane to provide the desired aircraft visualapproach path. This will likely result in tiltswitch module 10 beingtilted off the horizontal. To compensate for this tilt and to bringL-bar 27 into a horizontal position wing-nut 13 is loosened and module10 is rotated about pivot 12 until the upper surface of module 10, andhence, L-bar 27 is absolutely level or horizontal. Bolt 14 moves in slotduring this set-up procedure and when the upper surface of module 10 islevel wing-nut 13 is tightened to secure the module in that position.

Pre-tilt adjustment screws 32 and 33 are adjusted so that mounting bar15 is tilted approximately two-thirds of a degree downward toward theend depressed by pre-tilt adjustment screw 33. Rotation is about fulcrumscrew 16. Pre-tilt adjustment screws 34 and 35 are adjusted so thatmounting bar 24 is tilted approximately three-eighths of a degreedownward towards the end engaged by pre-tilt adjustment screw 34.Rotation of mounting bar 24 is about a fulcrum screw, not shown,centrally located in mounting bar 24 as fulcrum screw 16 is located inmounting bar 15. The orientation of the mounting bars just describedassumes that the mounting bars lie in a plane having a directionparallel to a vertical plane bisecting the light beam emanating from theVASI lamps and that the lamps are radiating to the right of the tiltswitch module as shown in FIG. 2.

The respective pre-tiltings of mounting bars 15 and 24, as justdescribed, result in the breaking of the electrical circuit throughmercury-thallium switch 25 if the VASI lamps 63 are deflected upward 1degree and the opening of mercury-thallium switch 26 if the VASI lamps63 are deflected downward by one-half degree.

Going through a cycle of operation, when VASI lamps are properlydirected along a predetermined tilt or aiming line so as to provideproper aircraft visual approach information and there are no externaltransient vibrational forces being applied to the assembly includingmodule 10 and VASI lamps 63, mercury-thallium switches 25 and 26 areclosed applying a D. C. potential to junction 77 between resistor 55 andcondenser 56 and junction 78 between resistor 51 and condenser 52,respectively, charging condensers 56 and 52 through resistor 49,conductor 76 being the common or negative side of D. C. power supply 67.

When transient vibrational forces having magnitudes sufficient toproduce vibrations of the VASI lamps and tilt-switch module 10 up to 1inch in amplitude at frequencies from one-half cycle per second to 10cycles per second, mercury-thallium switches 25 and 26 open alternatelyremoving the D. C. potential from junctions 77 and 78, respectively.Relay contacts 59 and do not open instantaneously, however, because ofthe discharge of condensers 52 and 56, respectively through resistors 51and 55 and pull-in windings 53 and 57, respectively. The time for suchdischarge exceeds the time mercury-thallium switches 25 and 26 are openas a result of transient vibrations which have been described. As aresult power to pull-in winding 61 of power relay 62 is not interruptedand the VASI lamps 63 continue to operate. On the other hand, if theaiming angle or desired tilt of VASI lamps 63 is significently disturbedon a steady state basis for in excess of a maximum of 1.5 minutes eithermercury-thallium switch 25 or 26 will open and the potential will beremoved from either junction 77 or 78. The corresponding condenser willdischarge through its associated pull-in winding in a time determined bythe magnitude of its associated resistor 55 or 51, respectively. Thedischarge of condensor 56 or 52 will result in a cessation of currentflow through pull-in windings 57 or 53, respectively, and contacts 60 or59 will open interrupting the flow of current through pull-in coil 61 ofrelay 62, contacts 64 will open and VASI lamps 63 will be extinguished.

As described earlier in this specification the VASI system comprises aminimum of two light bars spaced upwind and downwind along a runway. Ifeither light bar suffers a long term deflection from its predeterminedaiming angle, it is essential that both light bars or sets of VASI lampsbe extinguished so that a pilot will not have erroneous approachinformation. One light bar and its associated safety system have beendescribed. Additional light bars have corresponding tilt-switch modulesand associated circuits to assure extinguishment of the lghts in thelight bar when a longterm displacement occurs but not when a short termdisplacement from a transient phenomenon occurs. To make each light barsoperation dependent not only on its own proper aiming angle but also onthe proper aiming angle of any associated light bar it is only necessaryto place the pull-in windings of the power relays in the respectivelight-bar assemblies, such as pull-in winding 61 of relay 62, in serieswith each other and with the contacts 59 and 60 and their counterpartsin the other light bar assemblies, across a source of A. C. powerinstead of connecting the pull-in coil of each power relay through onlyits associated relay contacts, such as 59 and 66, of its localtilt-sensing circuit directly to an A-C power source, as shown in FIG.2. Thus if any light bar assembly is deflected on a long term basis fromits desired tilt or aiming direction the tilt-switch module in thatassembly will cause the pull-in current for the power relay in thatassembly and in any other interconnected assemby to be interruptd, thecontacts on that relay and on all other inter-connected power relayswill be opened and the current to both the local VASI lamps and those inthe other light bar or light bars will be interrupted assuring thatapproaching aircraft will not be given erroneous visual glide pathinformation. At the same time because of the controlled reaction time ofthe relays such as relays 54 and 58 transient displacement arising fromvibration of the light bars during jet blasts or the like will not causedisabling of the VASI lamps and the visual approach system will continueto be available to pilots landing at the airport.

A suggested set of component values for the circuit of FIG. 2 are asfollows:

Resistor 68 1.2K ohms Resistor 49 LSK ohms Resistor 50 7.5K ohmsResistor Ell 7.5K ohms Resistor 55 7.514 ohms Condenser 73 93microfarads Condenser 56 I00 microfarads Condensor 52 100 microfaradsWhile a particular embodiment has been described, modifications may bemade within the scope of the invention. The following claims areintended to cover such embodiments.

What is claimed is:

ll. An airport runway visual approach slope indicator including lightbar means aimed at predetermined tilt angles and a safety system; afirst set of terminals for providing electrical current to said lightbar means; a second set of terminals for providing operating power tosaid safety system; said safety system including electrical switches,sensitive to their angles of tilt, physically associated with said lightbar means and responsive to short term or long term changes in said tiltangles of said light bar means to open or close the electrical currentpaths through said electrical switches; at least one D. C. power source,first relay means, including a plurality of relays and second relaymeans associated with said light bar means; said first relay means beingcoupled between said at least one D. C. power source and said electricalswitches and having first and second conditions; said second relay meansbeing coupled to said first relay means and being responsive to saidfirst condition thereof to couple said first set of terminals to saidlight bar means and being responsive to said second condition thereof todisconnect said light bar means from said first set of terminals; andtime delay means coupled to said first relay means to prevent changingthereof from said first condition to said second condition in responseto the opening of any of said electrical switches for less than apredetermined period of time.

2. Apparatus according to claim l in which said time delay meanscomprises the serial combination of a resistor and a condensor shuntingeach of the relays in said first relay means.

3. Apparatus according to claim 2 in which said electrical switches eachhave a pre-adjusted initial tilt angle off the horizontal.

4. Apparatus according to claim 3 in which said electrical switches arethallium-doped mercury switches.

5. Apparatus according to claim l in which said electrical switches aretwo in number, one being responsive to excessive upward tilt of saidlight bar means the other being responsive to excessive downward tilt ofsaid light bar means to open the electrical circuit through therespective one of said switches; said first relay means comprising twoD. C. operated relays each having a set of contacts and a pull-in coileach said pullin coil being serially coupled through its respective oneof said electrical switches to said at least one D. C. power source;said second relay means including a pullin coil and at least one set ofcontacts; said pull-in coil of said second relay means being connectedserially with said set of contacts of each of said D. C. relays in saidfirst relay means across said second set of terminals; said at least oneset of contacts of said second relay means being connected in serieswith said light bar means across said first set of terminals.

6. Apparatus according to claim 1 in which said light bar means includestwo light bars separated from each other a predetermined distance alongan airport runway, each said light bar having a pre-set tilt angle; saidsafety system including a pair of mercury-thallium switches mounted eachwith a predetermined initial tilt angle in mechanical connection witheach of said light bars; a D. C. power source associated with each lightbar; said first relay means including first and second D. C. operatedrelays associated with each light bar, each said relay having a pull-incoil connected through a respective one of said mercury-thalliumswitches to said D. C. power cource associated with its respective lightbar andeach said relay having a set of normally open contacts; saidsecond relay means including a power relay associated with each of saidlight bars, each said power relay having a pull-in coil and at least oneset of normally open contacts; said second set of terminals beingadapted for the application of A. C. voltage thereto and being connectedserially through said contacts of said first and second D. C. operatedrelays to said pull-in coil of the power relay associated with thecorresponding light bar; said at least one set of normally open contactsof each of said power relay being connected in series between said firstset of terminals and said two light bars.

'7. Apparatus according to claim 6 in which one of said mercury-thalliumswitches has a predetermined tilt angle so as to make it responsive toan upward change 8. Apparatus according to claim 1 in which said time ofla in f tilt 38 of Said light bar means to delay means produces a delayof at least 1 second in the the electrical circuit therethrough and theother of said Chan of Said first rela means from Said first condimercury-thallium switches is responsive to a downward gmg y change of 16in said tilt angle of said light bar means tion to Said Secondconditionto open the electrical circuit therethrough.

1. An airport runway visual approach slope indicator including light barmeans aimed at predetermined tilt angles and a safety system; a firstset of terminals for providing electrical current to said light barmeans; a second set of terminals for providing operating power to saidsafety system; said safety system including electrical switches,sensitive to their angles of tilt, physically associated with said lightbar means and responsive to short term or long term changes in said tiltangles of said light bar means to open or close the electrical currentpaths through said electrical switches; at least one D. C. power source,first relay means, including a plurality of relays and second relaymeans associated with said light bar means; said first relay means beingcoupled between said at least one D. C. power source and said electricalswitches and having first and second conditions; said second relay meansbeing coupled to said first relay means and being responsive to saidfirst condition thereof to couple said first set of terminals to saidlight bar means and being responsive to said second condition thereof todisconnect said light bar means from said first set of terminals; andtime delay means coupled to said first relay means to prevent changingthereof from said first condition to said second condition in responseto the opening of any of said electrical switches for less than apredetermined period of time.
 2. Apparatus according to claim 1 in whichsaid time delay means comprises the serial combination of a resistor anda condensor shunting each of the relays in said first relay means. 3.Apparatus according to claim 2 in which said electrical switches eachhave a pre-adjusted initial tilt angle off the horizontal.
 4. Apparatusaccording to claim 3 in which said electrical switches arethallium-doped mercury switches.
 5. Apparatus according to claim 1 inwhich said electrical switches are two in number, one being responsiveto excessive upward tilt of said light bar means the other beingresponsive to excessive downward tilt of said light bar means to openthe electrical circuit through the respective one of said switches; saidfirst relay means comprising two D. C. operated relays each having a setof contacts and a pull-in coil each said pull-in coil being seriallycoupled through its respective one of said electrical switches to saidat least one D. C. power source; said second relay means including apull-in coil and at least one set of contacts; said pull-in coil of saidsecond relay means being connected serially with said set of contacts ofeach of said D. C. relays in said first relay means across said secondset of terminals; said at least one set of contacts of said second relaymeans being connected in series with said light bar means across saidfirst set of terminals.
 6. Apparatus according to claim 1 in which saidlight bar means includes two light bars separated from each other apredetermined distance along an airport runway, each said light barhaving a pre-set tilt angle; said safety system including a pair ofmercury-thallium switches mounted each with a predetermined initial tiltangle in mechanical connection with each of said light bars; a D. C.power source associated with each light bar; said first relay meansincluding first and second D. C. operated relays associated with eachlight bar, each said relay having a pull-in coil connected through arespective one of said mercury-thallium switches to said D. C. powercource associated with its respective light bar and each said relayhaving a set of normally open contacts; said second relay meansincluding a power relay associated with each of said light bars, eachsaid power relay having a pull-in coil and at least one set of normallyopen contacts; said second set of terminals being adapted for theapplication of A. C. voltage thereto and being connected seriallythrough said contacts of said first and second D. C. operated relays tosaid pull-in coil of the power relay associated with the correspondinglight bar; said at least one set of normally open contacts of each ofsaid power relay being connected in series between said first set ofterminals and said two light bars.
 7. Apparatus according to claim 6 inwhich one of said mercury-thallium switches has a predetermined tiltangle so as to make it responsive to an upward change of 1* in said tiltangle of said light bar means to open the electrical circuittherethrough and the other of said mercury-thallium switches isresponsive to a downward change of 1/2 * in said tilt angle of saidlight bar means to open the electrical circuit therethrough. 8.Apparatus according to claim 1 in which said time delay means produces adelay of at least 1 second in the changing of said first relay meansfrom said first condition to said second condition.